The BIRKENSTOCK Sierra is the perfect sandal for those that are on their feet throughout the day and want shoes that provide exceptional comfort. The Adventure Crosstown is a sporty version of this model. It is designed with all adventurous and active BIRKENSTOCK fans in mind. The polyurethane sole is foamed onto the footbed in two layers, which results in excellent non-slip properties, great flexibility and a long life. The upper is made of especially thick, oiled nubuck leather with unfinished edges.